This invention relates to inductive conductivity sensors for measuring conductivity of a sample fluid, and particularly to inductive conductivity sensors having a diagnostic resistor in parallel with a current path in the fluid.
Inductive conductivity sensors are used for measuring the conductivity of a fluid, such as a liquid or dispersion of solids suspended in the liquid. Inductive conductivity sensors are used to investigate the properties of electrolytes in the fluid, such as the degree of disassociation, the formation of chemical complexes, and hydrolysis.
A toroid inductive sensor is a common form of inductive conductivity sensor that employs two spaced-apart xe2x80x9ctoroidxe2x80x9d transformer coils. A drive coil is electrically excited by an alternating current source to generate a changing magnetic field. The changing magnetic field induces a current loop in the sample fluid; the magnitude of the induced current is indicative of the conductivity of the fluid. A detection coil inductively detects the magnitude of the induced current and provides a measure of the conductivity of the fluid. The body of a toroid sensor is typically cylindrical, and the coils are positioned near opposite ends of the cylinder. The axial passage of the cylinder defines part of the induced current loop in the fluid.
One problem associated with inductive conductivity sensors is that an open circuit condition in either the drive or detection coil circuits is difficult to detect. More particularly, an open circuit condition in the drive coil circuit results in no current being induced in the fluid. However, ion content of the fluid may generate noise in the detection coil that may be mis-analyzed as a conductivity value. An open circuit condition in the detection coil circuit results in no current being supplied to the analyzer from the detection coil, which might be mis-analyzed as a highly resistive (zero conductivity) fluid. The present invention is directed to a technique that permits diagnostics to be performed on the sensor to detect an open circuit condition in either the drive or detection coil electronics.
An inductive conductivity sensor for measuring conductivity of a fluid includes a housing arranged to be positioned adjacent the fluid. The housing supports a controlled impedance loop and a transducer. The transducer includes a driver arranged to induce a first current into fluid adjacent the housing and to induce a second current into the controlled impedance loop. The transducer also includes a detector arranged to inductively measure the first and second currents.
The current measured by the detector comprises the current representative of solution conductance (the first current) offset by the induced in the controlled impedance loop (the second current). The offset or second current provides the advantage of sensing open circuit in the sensor circuits, as well as to offset the detector current to overcome noise.
In some embodiments, the controlled impedance loop includes a conductive wire inductively coupled to the driver and the detector and a resistor coupled to the conductive wire.
In one form of the invention, the driver includes a first magnetic core adjacent a first end of the housing and a first coil arranged around the first core. Similarly, the detector includes a second magnetic core adjacent a second end of the housing and a second coil arranged around the second core. The controlled impedance loop extends through the first and second cores.